Astigmatism after cataract extraction: 6-year follow up of 6.5- and 12- millimeter incisions.

BACKGROUND

Cataract surgery and cataract surgeons have long realized that the surgical wound induces a significant change in the astigmatic properties of the eye postoperatively. In efforts to avoid or minimize these changes, there has been a progressive shift toward smaller incision sizes. However, that incision size which renders the eye astigmatically neutral has yet to be completely defined. This article will examine the long-term behavior of planned extracapsular surgical wounds (chord length 12 mm) and phacoemulsification wounds (chord length 6.5 mm), as well as look at potential animal models for defining how smaller incisions behave.

METHODS

One hundred thirty-eight eyes of patients whose average age was 75 years have been followed for up to 6 years. Both vector and scalar analyses of the induced postoperative cylinder were evaluated. Additionally, the astigmatic properties of 10 nonhuman primate eyes which had phacoemulsification surgery were followed for 1 year. Finally, incisions of smaller chord lengths, as small as 2.5 mm, were examined in the nonhuman primate model.

RESULTS

We demonstrated that with vector analysis, the induced cylindrical change with phacoemulsification wounds (6.5 mm) was 1.20 +/- 0.60 diopters and with planned extracapsular wounds was 2.20 +/- 1.30 D. In all cases, a significant against-the-rule shift was observed which was independent of the amount of surgically induced with-the-rule astigmatism at the time of surgery. Vector analysis was more appropriate because it demonstrated the against-the-rule shift for both patients whose preoperative cylinder was "with-the-rule" as well as those who were preoperatively "against-the-rule." In humans with 6.50-millimeter scleral incisions, these changes were completely stable 2 to 6 months postoperatively depending on the cylinder induced at the time of surgery. Experiments were also carried out in monkeys demonstrating that similar types of surgical wounds induced roughly the same amount of induced against-the-rule cylindrical shift (1.40 +/- 1.00 D), suggesting that this model is appropriate for looking at the behavior of cataract wounds in general. Smaller wounds in the nonhuman primate showed that 3.5- and 2.5-millimeter wounds were astigmatically neutral whereas 4.5-millimeter wounds and larger were not. Although the later study was performed with corneal incisions, whereas the studies mentioned earlier were all performed with scleral incisions, it is felt that this model could define that incision size which is neutral and that further work utilizing this model with other wound locations might be clinically helpful.

CONCLUSIONS

Although the long-term behavior of 6.5-millimeter phacoemulsification wounds was generally stable, a small clinically insignificant shift did occur several years after the initial surgical procedure (0.30 D). Animal modeling, although difficult and costly, could provide relevant data in defining the specific size and placement of surgical wounds for cataract surgery.